High-frequency alternating current transformer



Nov. 17, 1953 R. CALVERT 2,659,345

HIGH-FREQUENCY ALTERNATING CURRENT TRANSFOBMER Filed Feb. 15, 1950 4Sheets-Sheet l NOV. 17, 1953 c v R 2,659,845

HIGH-FREQUENCY ALTERNATING CURRENT TRANSFORMER Filed Feb, 13, 1950 4Sheets-Sheet I5 I DYM- 4 E Patented Nov. 17, 11953 UNITED STATES PATENTOFFICE HIGH-FREQUENCY ALTERNATIN G CURRENT TRANSFORMER ApplicationFebruary 13, 1950,'Serial No.'143-,975*

12.0laims. 1

This invention relates to alternating current transformers for. use. athigh frequencies and particularly totransformers for use in alternatingcurrent bridge circuits of the kind in which two windings on atransformer form the ratio arms of the bridge.

High frequency transformers for alternating current bridges haveheretofore been usually constructed with toroidal windings on a ringcore, the core being ofv rectangular cross-section built upof a numberof layersof high permeability taper It is the usual practice for the.primary winding to be wound toroidally. on the core extending around theWhole length of the core. Overthis windingis fitted a copper screeningring which covers the entire core and primary winding but is slit alongthe length of its face around the inside of the ring to avoid forming ashort circuitedturn. The secondary windings, comprising one or moreturns. of copper tape spaced with insulation of oneor two thousands ofaninch thickness are arranged on the outside of the screening ..ring..The copper tape has to be wound on a flat surface whereas the. ring iscurved along its length and therefore, to form a suitable foundation onwhich to wind the tape, a former is .built up to a rectangular orcircular cross-section by winding on the screening ring a number ofturns of suitableinsulating material.

One of the main electrical disadvantages of this form of construction isthat it causes a large amount of. leakage inductance; this is partly dueto the air spaces between the secondary winding and the screening. ringthrough which leakage flux can flow and partly due to the unnecessarilylarge periphery of the. secondary winding both of which features areconsequent upon the necessity of building up a former on which to windthe flat copper tape. In the kind of bridge circuits mentioned above,leakage inductance is highly undesirable. It may be reduced, by variousexpedients, 'buteven then is far from negligible at high radiofrequencies. Furthermore, .any form of construction whichiis adopted. tokeep down the leakage inductance tends :to raise'the winding capacity,so that the windings, innaddition'to their, normal circuit currents,also carry appreciable capacity currents. These in turn set up leakagefluxes which, unless correctly proportioned, can lead .to serious errorsin measurements at high frequencies.

The object of the present invention is to provide a transformer for useat high frequencies in which. (a) leakage inductance is reduced to aminimum, (b) the residual leakage inductance 2 can be distributed in anydesired waybetween two or more windings, and (c) the voltage deviationsbetween windings, due to the interaction of winding capacity on leakageinductance, can be brought under control.

These three requirements are achieved in the following. order. First,the overall construction of the transformer is dealt with; it-ismade'physically as small as possible, air spaces are eliminated, theperipheries of windings are reduced, and so forth, all with-the objectofbringing the leakage inductance to aminimum; Secondly, the requireddistribution of residual leakage inductance between windings isapproximated by a suitable disposition of turns". Thirdly, withoutchanging the relative positions of-Wi-ndings, permutation of theindividual turns. of the windings is made to establishthe optimum ratioof reactive volts due to capacity currents.

Further features of the inventionwill be' apparent from the followingdescription of-a number of embodiments thereof reference being made tothe accompanying drawings in which Figure l is a front View of a highfrequency transformer of the kind used heretofore, with part of thescreening ringv and secondary winding former cut away;

- Figure 2 is a side view of the transformer of Figure 1 with. thetop-half in section along the line 2-2 of Figure 1;

Figure 3. is a part view of a high frequency transformer constructedaccording to the present invention with'part. of the screening ring.-and secondary Winding: former cut away;

Figure. 4 is aside View of the transformer of Figurev 3 with the tophalf in' section along. the line 4 of Figure3 Figures 5 and 6 are viewsrespectively similar to Figures 3 and act a .second embodiment of theinvention;

Figure 7 is a diagrammatic section through the Secondary winding of. atransformer, the spacing between layers being exaggerated;

Figure 8 is-a perspective view of another construction of transformer;

Figure 9 is-a diagrammatic section through the windings of thetransformer shown in Figure 8,

Figure 10"i's a diagram illustrating the transformer windings Figurellisa diagrammaticillustration of the secondary windingscut and opened outflat;'

Figures 12 (a); ('bD',-("c) and-Cd) are diagrams illustrating thecurrents in the various turns of the secondary windingsandthere'sultants M. F.s,

Figure 13 illustrates the method of folding the tapes of the secondarywindings; and

Figures 14(a) and (b) and Figures 15(a), (b), (c) and (d), are diagramsillustrating the capacity currents in the secondary windings.

In order to understand the present invention more clearly, it isnecessary to refer to the types of transformer heretofore used. Atypical transformer is shown in Figures 1 and 2 and comprises a ringcore II] on which toroidally is wound a primary winding H which extendsaround the whole of the core. Over the primary winding is a copperscreening ring [2 which has a slit I3 extending around its inner surfaceto prevent the screening ring forming a short circuited turn. Thesecondary windings comprising one or more turns of copper tape M withinsulating spacing are placed on the outside of a former l5 which isbuilt up to a rectangular section by winding on the screening ring anumber of turns of suitable insulating material. Three tapes I6, ll, [8are lead out sideways from the winding providing connections to the twoends and to an intermediate point between the ends. As indicated above,the leakage inductance with this construction is relatively great duepartly to the air spaces indicated by the reference 19 and partly to thelarge periphery of the secondary winding.

Figures 3 to 9 show transformers constructed according to the presentinvention in which the air spaces have been eliminated and the secondarywinding periphery greatly reduced.

Referring to Figures 3 and 4, the core 20 is made D-shaped instead ofcircular. Regarding the copper screening ring as a coupling turn betweenthe primary and secondary windings, it is clear that there is nonecessity to place any part of the primary winding immediatelyunderneath the secondary wingings. The primary winding 21 is wound onthe curved part of the D-shaped core and the screening ring 22 is drawnin to become a close fit on the core along the straight part of the D.As is seen more closely from Figure 7 which is a diagrammatic sectionthrough the secondary winding with the spacing of the layersexaggerated, the secondary winding 23 formed of copper tape is thenwound over this straight part on top of a layer of insulation 24. Thiswinding is formed of a number of successive layers separated by thininsulation and is arranged in a manner to be described hereinafter.

Figures 5 and 6 show a construction which is generally similar to thatof Figures 3 and 4 and the same reference numerals have been used toindicate similar components. In this arrangement however, the screeningring 22 has been made a close fit on the core 20 except at one placewhere it is drawn out to accommodate the primary winding 2|.

Another construction is shown in Figures 8 and 9 in which a dust ironcore is formed of two L-shaped members 25. The secondary winding 26 iswound on top of the primary winding 21. No intermediate screening isused and in order to preserve the capacity balance between the primaryand secondary windings, an extra half turn 28 is disposed between thetwo windings, one end of this half turn being connected to the outer endof the primary winding 21. Preferably in this arrangement the outer endof the primary winding is earthed. The arrangement shown in Figures 8and 9 obviates the necessity for a screening ring and is particularlysuitable for very high frequency bridges.

By these arrangements the air spaces under the secondary windings areeliminated and the periphery of this winding has been greatly reduced.Having thus brought the leakage inductance to a minimum, the nextrequirement is to control the distribution of the residual leakagebetween the windings.

For ease of description the particular example of the currenttransformer in a high frequency admittance bridge such as that describedin my copending application Serial No. 143,976 filed February 13th,1950, now Patent No. 2,589,535, and entitled High Frequency AlternatingCurrent Bridges will be considered. The transformer is shownschematically in Figure 10 and comprises a first winding 30 (the primarywinding) connected to a detector and two secondary windings, one ofwhich referred to hereinafter as the standard winding comprises in thisparticular example three turns which will be called SI, S2 and S3 turnsand connects the standard impedance to the neutral connection. The othersecondary winding connects the unknown impedance to the neutralconnection and is therefore referred to as the unknown winding and itssingle turn will be called the U turn. In the type of bridge referred tothese two secondary windings form the ratio arms. Ideally the bridgeshould balance when the current through the unknown is three times thecurrent through the standard, and in the balance condition thetransformer terminals connecting the unknown and standard should both bebrought to the potential of the neutral connection, owing to thecancellation of fluxes in the two windings. In practice, of course,there must be leakage flux in the windings so that when the core fluxescancel, giving zero voltage on the detector winding, there will remainsmall reactive voltages at the standard and unknown terminals. Thesevoltages will cause errors in measurement unless they are correctlyproportioned so as to cancel in their effect.

Since the leakage inductances arise from the magnetic fluxes that flowin the spaces between turns, it is convenient for the calculation ofleakage inductance to imagine the windings cut through and opened outfiat. This is shown in Figure 11, which is greatly distorted to enablethe dimensions to be marked. The dimension line 1 indicates the lengthof a single turn of the winding, W indicates the width of the tapeforming the winding and d is the spacing between the successive layers.

As a particular example, it will be assumed that there is aninstantaneous current of 1 amp. in the standard winding and 3 amps. inthe unknown winding, in the sense shown in Figure 10. First, acalculation will be made of the leakage inductances when the singleturn, U. of the unknown winding is the inner turn, and the three turns,S1, S2 and S3, of the standard winding are placed on top of it. Thisstate of affairs is represented in Figure 12(a), which shows a crosssection of the cut and flattened windings of Figure 11 and is useful asan aid to calculation.

In the space (a) of Figure 12(a) there is a flux 31; units downwardsthrough the plane of the paper caused by the current in turn U. There isalso a flux in 11 units downwards through the plane of the paper due toeach of the currents in turns S1, S2 and S3. The result is as though thespace (a) was acted upon by an M. M. F. of

sampere'tu'fns; In spaceflbithere'is a flux of 31; units down throughthe plane of the paper due to "the current in U, and fluxes of 151 unitsdown through the plane of the paper due to the currents in S2 and S3;but a flux of 17 units up th'roiightheplane of the paper due tothe'current in turn S1. The resultant is as though the space was actedupon by a M. M. F. of 2 ampere turns. Similarly the flux in the space(0) is as though it was acted upon by aM. M. F. of l ampere turn. Thesefigures are marked in the spaces on the diagram. Knowingthe M. M. II, itis possible to calculate the reluctance of the flux path. From Figure itisclear that the reluctance of the portion of the flux path inside thewinding is In viewof the small dimension d'one or two mils only -it isclear thatthe reluctance of the fluxpath outside the winding isextremely small compared with the reluctance of the portion of the pathinside and, therefore, to a first approximation it can be ignored.Knowing the M. M. F.s and the reluctances of the flux paths, the fluxesinthe spaces can be written down as follows:

Flux in (b)='3.l9% v2 (dimensions in inches) The flux linkages for theturns are as follows: Flux linkages in turn U=0 Flux in (c) 3.19

The effective leakage inductance of the U windingis" clearly zero, sincethe single turn of this winding embraces no leakage flux. The effectiveleakage inductance of the S winding is found by dividing the total fluxlinkage for the three turns of the winding by the current. This gives:

Leakage inductance of the S winding=L,=3.19%

L -0 03l9 l4 h 3" W H y With this particular arrangement of winding itis clear that the whole of the effective leakage inductance occursin'theS winding, and that if itis desired to distribute it in some proportionbetween the S- and U windings, the U turn must beallbwe'd to embracesome of the leakage flux. Suppose; therefore that the winding bearranged as indicated in Figure 12(1)). That is to say S1 is made theinner turn, U the next turn and S2 and S3 the final two turns. Thecalculation is carried through as above. Defining positive fiux as fluxdownwards through the plane of the paper, it is first of all necessaryto reckon the effective M. M. F. acting on the spaces a, b, and cthese'are marked in the figure-then the space 6' fluxes andfluxlinkagesper"turn; Finally, the flux linkages are added, divided bythe current and multiplied by 10- The result is:

If the U turn is moved one more turn away from the core the arrangementis as shown. in Figure 12(0), and if it is placed completely outside theS winding, as shown in Figure R(d). A calculation of the leakageinductances gives the following:

For Figure R(c) Zd L,-0.0319- 3p. hy.

and for Figure 9(d) L,=0.0319%,4,u hy.

Leakage inductances have now been'calculated for a turn by turnwithdrawal of the U turn from the inside to the outside of the winding.It should be explained however that these'particular positions for the Uturn are chosen solely for simplicity of illustration. The withdrawalcan equally well be continuous; The windingscould, for instance, bearranged as follows:

1st layer %S1+ AU 2nd layer %U+ 481 3rd layer S2 4th layer S3 That is tosay, the U winding could be started turn from the core end; Indeed,there is no reason other than ease of physical construction why the Uturn itself should be put on as a con tinuous strip. In short, the twowindings can'be broken into sections and. the sections interleaved inany desired way to achieve a particular ratio of leakage inductances.

The necessary electrical continuity of the windings can be achievedeither by bringing out interconnecting tapes at right angles to thedirection of winding, or alternatively by cross-overs formed by foldingthe tapes as shown in Figure 13. In this figure there are shown twoconducting tapes marked X and Y with a layer'of insulation between them.By successively folding the tapes and insulation together in three foldsas illustrated, the order of the layers may be reversed i. e. the lowestlayer is brought to the top. It will be appreciated that by employing asuc cession of such cross-overs, any turn of the winding may be broughtout through all the overlying turns.

In many applications it is desirable to be able to trim the leakageinductanceratio to allow for stray inductances in theexternal circuit.This is conveniently done as follows. The transformer is so designedthat the leakage inductance of the winding that includes the outer turnis slightly below its estimated required value. The leakage inductanceof this winding can then be increased as required by increasing thespacing between the final turn and the rest of the winding over a part,or the whole of the periphery, as is illustrated diagrammatically inFigure 7 the spacing 40 between the final turn and the rest of thewinding is greater than the spacing 4| between the inner turns.

This same technique of increasing (or decreasing) the insulationthickness in the outer turns is sometimes useful in overcomingconstructional difficulties associated with fractional turninterleaving. For example, with a square winding cross section it isdifiicult to bring out interconnecting tapes at places other thanintegral multiples of a quarter turn. In such cases, if the requiredresult cannot be achieved by folding the tapes to give cross-overs, itis convenient to make the connections to the nearest quarter turn andthen to achieve the required leakage inductance ratio by suitablyvarying the insulation thickness.

For ease of description the discussion has been confined to oneparticular type of transformer, but the same principles can be appliedequally well by obvious extensions of the argument to many other types.

Having satisfied the first two requirements, it is now only necessary tobe able to control the eifects of leakage flux due to capacity currents.The discussion will again be confined to the particular example of thetransformer described above.

First, the winding arrangement of Figure 12(b) should be considered,where the U turn is one turn removed from the core end of the winding.It should be assumed that the transformer is disconnected from thecircuit on its U and S windings and excited on its detector winding soas to induce 1 volt per turn.

Let the instantaneous voltages be as indicated in Figure 14(a). Thewinding connections are as follows: The start of the U turn is an opencircuit and at zero voltage; there is a rise of voltage along the U turnto 1 volt at the end, which is connected to the start of the S1 turn;the finish of the S1 turn, which is at 2 volts, is connected to thestart of the S2 turn; the finish of the S2 turn, at 3 volts, isconnected to the start of the S3 turn; the finish of the S3 turn, at 4volts, is an open circuit.

Let the capacity between turns be C, which is given by the usualformula:

where K is the dielectric constant of the insulating material and thedimensions are in inches as indicated in Figure 11.

Between any point on the S3 turn and a point immediately below it on theS2 turn there is an instantaneous potential drop of 1 volt. Between anypoint on the S2 turn and the point immediately below it on the U turnthere is an instantaneous potential drop of 2 volts. Between any pointon the U turn and the point immediately below it on the S1 turn there isan instantaneous potential drop of minus 1 volt.

It is now necessary to consider a small element of length AZ of thecross section of the winding immediately to the left of the finish ofthe S2 turn. The capacity between the S3 and S2 turns for this elementof length is Al O The voltage across it is 1 volt, therefore a capacitycurrent Ai=jw C must flow from the element of the S3 turn to the elementof the S2 turn.

Since the finish of the S turn is an open circuit no current can flowinto the element from the right. It must therefore flow in from theleft. Continuing along the S3 turn to the left, every element of lengthAl withdraws a capacity current of and every increment of capacitycurrent must clearly flow from the left. Integrating along the turnshows that there is a linearly rising current in the tape to a maximumvalue of 9100 at the start. But the start of the S3 turn is connected tothe finish of the S2 turn. The finish of the S: turn must thereforecarry a current in its tape of disc flowing to the right. Proceedingback along the winding, that is to the left along S2, it is clear thateach element of length AZ receives a current of jw C from the S3 tapeabove it and gives out a current of to the tape below it. Integratingalong S2 to the left there is consequently, a linearly rising current toa value of 27'wc at the start of the turn. The start of the turn ishowever connected to the finish of the S1 turn, which must thereforealso carry a current of 271w flowing to the right. Each element of theS1 tape gives an increment of current to the U tape. Integrating alongS1 to the left shows a linearly rising current to a value of 37'wc atthe start. Finally integrating along the U tape, it is easily shown thatthe current falls linearly from a value of 3 at the end to zero at thestart.

The extreme value of currents are shown on Figure 14(a) with arrowsindicating the direction of instantaneous current flow.

A calculation is now made of the leakage fluxes resulting from thecapacity currents.

At a point x, where a: is measured from the start of a turn, theconductor currents are:

M. M. F. at :v in space (a) =jwc amp. turns M. M. F. at :c in space (b)=jwc amp. turns M. M. F. at x in space (0) jwc +2) amp. turns acumen 9"The reluctance of theflux' path over a; length Amsis 3L dzlrc sotheinstantaneous fluxes can be 'writtenas follows:

Instantaneous flux at :c in space (a) 4=rr dAx x d a:

I 2.54 ywc 3.19 7wc 1A1? Instantaneous flux at a: in space (b):

- BJQjw'ci Aa W l Instantaneous flux at :cin space.(c)=

d' a: '-3.lQ 7cJC +2)A$ (dimensions iu.inches) .The total instantaneousfluxes in the spaces (a), (b), and (c) are given by integrating throughthe range 0-1. Thisgives:

Total f ux in (a) =3 1900 f xdx=3 l9 wc 3 W1 J W 2 dl Total flux 1n(b)=3.19]wC 1 Total fin; in (0) =3 19 205 5 5 i W The instantaneous.flux linkages. on thefour conductors are therefore:

all 011 S33.IQJOJCW3 S elem-$0.5 U= 3.l9jwc% 0.5 w

The total instantaneous flux linkages of the S winding are:

3.1 9jwc%.3.5 and on the U winding =.19jwc%.0.5 The reactive volts onthe two windingsidue to capacity currents are therefore:

On S Winding=3.19w C-{% 3.5 10

dz On U Winding=3.19td C O.5 10- Replacing C by its value of wl.22elelK--1O 0 d 00 the reactive volts on the two windings resultingfrom the leakage flux of the capacity currents are as follows:

Reactive volts on S winding=3.5N 65 Reactive volts on U winding: 0.5Nwhere N=0.'716Kw l 10- It will now be shown how the reactive volts onthe winding due to capacity currents can be varied without alteringtheeffective leakage inductance of the windings.

In the transformer under consideration, the U winding has a single turnand the S winding has three turns. In calculating the leakage induct-"l0 ances of theitwo windings, a current or 1 amp. was'assumedin the Swinding and seams. in the 'U winding; and an estimate was made 0! theleakage fluxes in the spaces between turns. :III

5 the three turns S1, S2 and 33' each carry 1 amp.

of circuit current, it is clear that their positions can be interchangedwithout affecting the leakage inductances, provided only that thecorrect sense ofwinding is preserved. But interchanging their positionwill have a considerable effect on the capacity currents, as it willchange the values of induced voltage between turns without changing theactual inter-turn capacity. This will-be clear from Figures 15(a),15(1)), 15(0) and 15(d); whichshow four different arrangements of theturns S1, S2, S3. In each case the. turns are connected as follows: endof the U tum to start *of S1 turn; end of S1 turn to start. otS: turn;end of S2 to start of S3 turn; so that the leakage inductances areunaffected.

The reactive voltages resulting from the leakage flux of the capacitycurrents are calculated as above. They are:

For .the arrangement of Figure 15(a) .1 Reactive volts on S winding=3NReactive volts on U winding=2N For the arrangement of Figure 15 (b)Reactive volts on S winding=0.5N Reactive volts on U winding=1.5N

For thearrangement of Figure 15(c) Reactive volts on S winding=.2.5NReactive volts on U winding= 2.5N

For the arrangement of Figure 15(d) Reactive volts on S winding=-l0NReactive volts on U winding=-4N Another way of changing the distributionof leakage fluxesdue to capacity currents in the inter-turn spaces, andhence of changing the ratio of reactive volts induced in the twowindings by'capacity current, is by the introduction of dummy: turns, orfractional turns. This amounts'tothe introduction of a thirdwinding,connected to a suitablepoint on the-bridge winding at one. end'and opencircuited at theother. This will have: no effect on the leakageinductances'of the S and U windings (since it causes nocircuit currents)except insofar as it will change the effective inter-turn spacing if it.is introduced in the-middle of the windings ;*but it willxcarry. a'.capacity current depending in amplitudeand sense on its point ofconnection to the S' andU winding and its place of introduction .in' thewinding. The efiect is calculated by an identical'procedure to thatdescribed above. Such awdummy turn is arranged in a similar manner tothe :half turn 28 illustrated in Figure 9.

-Althougha number of specific embodiments of the invention have beendescribed and shown herein, it'willi'be understood that the details ofconstruction shown may be altered without departing from. the spirit ofthe inventionas defined byv the following claims.

I claim:

1. -A..high frequency. alternating current transformer comprising a.core member; aprimary winding extendingover one part of the core;:.ascreening member of.metal closely. fitting over and substantiallyenclosing both the core mem- .ber and'primary winding and a pairof-.secondary' windings' wound outside the screening mem- 11 her over apart of the core remote from the primary winding, one of the secondarywindings comprising a number of successive layers of a conductor and theother secondary winding comprising at least one turn disposed betweenthe layers of said one winding.

2. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion; a primary winding wound on part of the core remote from saidstraight portion; a screening member of metal substantially enclosingboth the core member and the primary winding which screening member hasa slit extending continuously along its length around the core and isarranged to fit closely over said straight portion of the core; and apair of secondary windings formed of metal tape wound over the screeningmember on said straight portion, one of the secondary windingscomprising a number of successive layers of tape and the other secondarywinding comprising at least one turn disposed between the layers of saidone winding.

3. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion; a primary winding wound on part of the core remote from saidstraight portion; and a pair of secondary windings formed of metal tapewound around the straight portion of the core, one of the secondarywindings comprising a number of successive layers of tape and the othersecondary winding comprising at least one turn disposed between thelayers of said one winding.

4. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion; a primary winding wound on part of the core remote from saidstraight portion; a pair of secondary windings formed of metal tapewound around the straight portion of the core, one of the secondarywindings comprising a number of superimposed layers of tape and theother secondary winding comprising at least one turn disposed betweenthe layers of said one winding; and insulating material disposed betweenthe successive layers of the secondary windings, the thickness of theinsulating material between the final turn and the penultimate turn overat least a part of the periphery being greater than the thickness of theinsulating material between the other layers.

5. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion; a primary winding wound on part of the core remote from saidstraight portion; a screening member of metal substantially enclosingboth the core member and the primary winding which screening member hasa slit extending continuously along its length around the core and isarranged to fit closely over the straight part of the core; a pair ofsecondary windings formed of metal tape wound over the screening memberaround the straight portion of said core, one of the secondary windingscomprising a number of superimposed layers of tape and the othersecondary winding comprising at least one turn disposed between thelayers of said one winding; and insulating material disposed between thesuccessive layers of the secondary windings, the thickness of the in.-

12 sulating material between the final turn and the penultimate turnover at least part of its periphery being greater than the thickness ofthe insulating material between the other layers.

6. A high frequency alternating current transformer comprising a coremember, a primary winding extending over one part of the core, a pair ofsecondary windings Wound over another part of the core remote from theprimary winding, one of the secondary windings comprising a number ofsuccessive layers of a conductor and the other secondary windingcomprising at least one turn disposed between the layers of said onewinding, and insulating material disposed between the successive layersof the secondary windings, the thickness of the insulating materialbetween the final turn and the penuitimate turn over at least a part ofthe periphery being greater than the thickness of the insulatingmaterial between the other layers.

7. A high frequency alternating current transformer comprising a coremember, a primary winding extending over one part of the core, and apair of secondary windings wound over another part of the core remotefrom the primary winding, one of the secondary windings comprising anumber of successive layers of a conductor the successive turns beingconnected in series in an order differing from the order in which theyare wound on the core to minimize the effect of the winding capacity onthe leakage inductance and the other secondary winding comprising atleast one turn disposed between layers of said one winding.

8. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion, a primary winding wound on part of the core remote from saidstraight portion, and a pair of secondary windings formed of metal tapeWound around the straight portion of the core, one of the secondarywindings comprising a number of successive layers of tape the successiveturns being connected in series in an order differing from the order inwhich they are Wound on the core to minimize the effect of the windingcapacity on the leakage inductance and the other secondary windingcomprising at least one turn disposed between the layers of said onewinding.

9. A high frequency alternating current transformer comprising a coremember shaped to form a closed magnetic circuit and having a straightportion; a primary winding wound on part of the core remote from saidstraight portion; a screening member of metal substantially enclosingboth the core member and the primary winding which screening member hasa slit extending continuously along its length around the core and isarranged to fit closely over said straight portion 1" the core; and apair of secondary windings formed of metal tape wound over the screeningmember on said straight portion, one of the sec-- ondary windingscomprising a number of successsive layers of tape the successive turnsbeing connected in series in an order diiiering from the order in whichthey are wound on the core to minimize the eiTect of the winding caacity on the leakage inductance and the other secondary windingcomprising at least one turn disposed between the layers of said onewinding.

10. A high frequency alternating current transformer comprising a coremember, a primary winding extending over one part of the core. a pair ofsecondary windings wound over the primary winding, one of the secondarywindings comprising a number of successive layers of a conductor and theother secondary winding comprising at least one turn disposed betweenthe layers of said one winding, and. a fourth winding connected at oneend to a point on the primary winding and open circuit at the other end,which fourth winding extends over at least a fraction of a turn and isdisposed between the primary and the secondary windings.

11. A high frequency alternating current transformer comprising a coremember and a plurality of windings including a first winding having anumber of successive layers of a conductor, a second winding disposedbetween the layers of said one winding and a compensating windingconnected at one end to a point on one of the other windings and opencircuit at the other end, to minimize the effect of the winding capacityon the leakage inductance, which compensating winding is disposedbetween the layers of the other windings.

12. A high frequency alternating current transformer comprising a coremember, a primary winding of conducting tape wound on said core, a pairof secondary windings of conducting tape wound over the primary winding,one of the secondary windings comprising a number of successive layersof tape and the other secondary winding comprising at least one turndisposed between the layers of said one winding, and a fourth windingextending over at least a fraction of a turn between the primary andsecondary windings with one end connected to the primary Winding and theother end open circuit.

RAYMOND CALVERT.

